Capacitors having a dielectric material made exclusively of synthetic resin are generally superior to those capacitors, which employ in addition to synthetic foils also paper layers between the electrodes, or which have a dielectric material consisting exclusively of paper. Field strengths of up to 100-120 V/.mu. are obtained in well impregnated capacitors before glow discharge sets in using foils exclusively at a usual strength of the dielectric material between 2 and 20 .mu. versus field strengths between 55 to 75 V/.mu. using a mixed foil construction, and 30 to 40 V/.mu. of field strength using exclusively paper foils as the dielectric material. The corresponding insulation breakdowns amount to 400 V/.mu., 300 V/.mu. and 150 V/.mu. respectively. The clear-cut superiority of the employment of synthetic resin films in the construction of a dielectric appears to be primarily due to the fact that they contain gas or conductive particles to only small measure, along which currents may form at high voltages. It is in any case certain, that the operating field strength in capacitors employing foils exclusively can be increased in comparison to paper capacitors to an extent, that one half of the volume can easily be saved. The avoidance of any intermediate paper layers, does not, however, solely relate to savings in volume and material, but it provides the additional advantage, that the necessary air evacuation from the capacitor roller is not unduly extended by the necessary to reudce the hight moisture content of the paper.
In spite of the known advantages of the capacitors employing exclusively foils, the latter have not in fact been used in practice up to now. This is due to the fact that an impregnation of a wound capacitor consisting merely of synthetic foil and starting at a foil width of approximately 10 cm is only possible without using an intermediate paper layer acting as a wick, if one ensures that adjoining foils, or foils and electrodes are in limited surface contact. In this sense several pre-treatments of the synthetic foil or metal electrodes have been proposed, such as roughening, stamping or embossing, partial shrinking and the like. These methods are not only costly, but there is constantly the risk that the deformation of the extremely thin foils could produce tears or irregularities in the latter, which would negate the desired success.